The Third Generation Partnership Project (3GPP) has developed standards for a cellular communications system, referred to as Long Term Evolution (LTE). LTE uses a Radio Access Network (RAN) based on Orthogonal Frequency Division Multiplexed (OFDM) communication. It also includes a System Architecture Evolution (SAE) Core Network. This comprises a Mobility Management Entity (MME) node, which provides control functionality for the RAN and a Serving Gateway for provision of user data to the RAN.
The LTE RAN (also referred to as E-UTRAN) and SAE Core Network provide all services using packed-switched communications, particularly using Internet Protocol (IP). Providing circuit-switched services, such as voice connections, is therefore a challenge in the LTE system, because service-specific structures are desirable to guarantee a minimum Quality of Service (QoS).
A number of architectural frameworks for handling specifically voice calls have been explored. These include: IP Multimedia Subsystem (IMS); Circuit Switched Fall Back (CSFB); Voice over LTE Generic Access (VOLGA); and Camping voice devices on GSM EDGE Radio Access Network (GERAN) or UMTS Radio Access Network (UTRAN). These and other systems for voice call handling are documented in “3rd Generation Partnership Project; Study on Circuit Switched (CS) domain services over evolved Packet Switched (PS) access; Stage 2” (3GPP TR 23.879 V9.0.0), which is incorporated herein by reference.
This multitude of implementations is a difficulty for designers of network and user equipment. Equipment manufactures and network operators would like to reduce the number of architectural frameworks for which they provide functionality.
One of the more favoured frameworks is CSFB, which is specified in ETSI TS 123 272 V8.4.0, which is incorporated herein by reference. This permits a mobile device to receive service normally from an LTE RAN. However, the architecture pushes the mobile device onto a GERAN or UTRAN system to provide voice call service.
However, there are several drawbacks to this framework. One particular challenge is that there can be a significant delay in setting up a circuit-switched call using this approach. This is due to the additional communication requirements between the LTE RAN and GERAN or UTRAN system and the need for the mobile device to establish communication with the GERAN or UTRAN system. Documents S2-095143 and S2-095144 from the 3GPP TSG-SA WG2 Meeting number 75, which are incorporated herein by reference, discuss some of the performance disadvantages of CSFB and propose some improvements which have the potential to reduce call set up delay and improve call setup reliability.
Another difficulty is that the Short Message Service (SMS) is not easily provided using the CSFB framework. This problem was addressed in WO-2009/056932, which is incorporated herein by reference. This implementation made use of the interface between the MME of the SAE Core Network and a Mobile Switching Centre (MSC) to provide SMS. The MSC provides legacy services to the LTE RAN and to the GERAN and UTRAN systems. The interface between the MME and MSC was originally intended for Paging and Mobility Management signaling. However, by extending this interface to provide SMS from the MSC to MME and using Non-Access-Stratum (NAS) signaling support between the mobile device and MME, full SMS services can be provided over the LTE RAN. Nevertheless, in view of the restrictions in the interface between the MME and MSC, referred to as the SGs interface, other higher bit-rate circuit-switched traffic cannot easily be communicated using this approach.
A further issue is that the CSFB approach requires that the mobile device can communicate with a GERAN or UTRAN system. This may not be possible in some cases, because the GERAN or UTRAN system may not have sufficient capacity to provide an additional service. Also, LTE networks may be operated in a frequency band around 800 MHz, which is lower than that used by known GERAN or UTRAN systems. Radio propagation is improved at this frequency. In conjunction with the more efficient use of radio bandwidth by the LTE RAN (for example, the use of OFDM and multiple antennas), it may provide radio service coverage over a wider area than other systems. Consequently, a mobile device served by an LTE RAN may not be able to receive service from another network.
It is therefore desirable to provide circuit-switched services, especially voice calling, in an efficient way over the LTE RAN, without the need for significant modifications to the RAN and Core Network.,